Method for determining the speed of a wheel on a motor vehicle

ABSTRACT

There is described a method for determining the speed of a wheel on a motor vehicle, for which purpose an acceleration sensor is mounted on the wheel. It is proposed according to the invention to measure the frequency ω or the period T of an alternating signal contained in the acceleration signal supplied by the acceleration sensor and produced by the influence of gravitational acceleration g. The method is suited for use in automatic tyre-pressure monitoring systems for assigning signals, that have been transmitted by radio from a wheel-mounted electronic module, to a particular wheel position and for controlling the transmission rate of a wheel-mounted electronic module as a function of the speed.

This application is filed under 35 U.S.C. §371 based upon InternationalApplication No. PCT/EP0/1178 filed on Feb. 3, 2001.

The present invention relates to a method for determining the speed of awheel on a motor vehicle, for which purpose an acceleration sensor ismounted on the wheel. The necessity to determine the speed of a wheelarises for motor vehicles with wheels equipped with pneumatic tyreswhose pressure is to be continuously monitored also in operation of thevehicle. DE 196 08 478 A1 describes for this purpose a tyre-pressuremonitoring system where each wheel of a motor vehicle is equipped with adevice (hereinafter referred to as wheel-mounted electronic module) thatcomprises a pressure sensor, an electronic evaluation sensor connectedto the pressure sensor, a transmitter, a sending antenna and a batteryfor supplying the wheel-mounted electronic module with current.Neighbouring the wheels, there are mounted on the vehicle body receivingantennas connected by cable to a receiver which processes and evaluatesthe signals transmitted to it by the receiving antennas and provides asignal to the driver when a substantial, especially a dangerous, changein pressure is observed on any of the wheels, in which case the driveris simultaneously informed on the particular location of the wheel inquestion.

One problem encountered in connection with such tyre-pressure monitoringsystems relates to the clear assignment of the individual transmittersto the location of the respective wheel on the vehicle. The transmittersgenerate for this purpose a signal that is subdivided into severalsections and that comprises a preamble, an identification signal, ameasurement signal and a postamble. The receiver is to determine by wayof the identification signal (hereinafter referred to as identifier) thelocation where the sending wheel is mounted on the vehicle. This is,however, possible only if the identifier and the wheel position havebeen clearly allocated one to the other before and if such allocationhas been stored in the receiver. DE 196 08 478 A1 discloses a way how toautomatically determine such allocation from the signals transmitted bythe wheel-mounted electronic modules after the first assembly of wheelson the vehicle and after every change of wheels. One performs for thispurpose a statistical evaluation of the intensity of the signalsreceived. Although each receiving antenna receives signals from alltransmitting wheels of the vehicle, it is assumed that, regardedstatistically, the signals received from the wheel that is the nearestto the respective receiving antenna will be received with the highestintensity on average.

A similar allocation method has been known from DE 196 08 479 A1.

Another way of automatically allocating the identifiers transmitted bythe wheel-mounted electronic modules to the respective wheel locationsis disclosed by DE 197 35 686 A1. This document proposes to design thewheel-mounted electronic module on the respective wheel in such a waythat it will determine not only the tyre pressure but also additionalinformation on the moving state of the wheel and transmit both data byradio to the central receiver in the motor vehicle. The moving state ofthe vehicle is used by the receiver to derive information on thelocation of the respective wheel on the vehicle. To this end, anyaccelerations encountered on the wheel are determined by accelerationsensors provided in the wheel-mounted electronic module, and either theacceleration data is evaluated in the wheel-mounted sensors provided inthe wheel-mounted electronic module, and either the acceleration data isevaluated in the wheel-mounted electronic module, in which case theresult of such evaluation is transmitted to the receiver by radio, orthe acceleration signals are inserted by the wheel-mounted electronicmodule into the signal that is to be sent out regularly and thatcontains a data telegram and is sent, together with such signal, byradio to the receiver where it is evaluated.

The known system permits information on the wheel position to be derivedfrom acceleration signals obtained on the wheel in the following ways:

1. When the wheel turns, a centrifugal acceleration is encountered onthe wheel the strength of which depends on the speed of the wheel. Thespeed of spare wheels carried with the vehicle is equal to zero, evenwhen the vehicle is moving. Any signals that are received when thevehicle is moving and that signal a speed equal to zero or a centrifugalacceleration Z equal to zero will therefore be allocated to a sparewheel carried with the vehicle.

2. When the speed or the value of the centrifugal acceleration Z isintegrated over a predetermined period of time, then the value of theintegrated measurand is a measure of the distance the wheel hastravelled during that period of time. Given the fact that when corneringthe steered front wheels travel a longer distance than the unsteeredrear wheels of a vehicle, the integrated measurand for a steered frontwheel will be greater than that for an unsteered rear wheel. Theidentifiers contained in signals that lead to the greatest integratedmeasurands for the speed or the centrifugal acceleration Z can thereforebe assigned to the steered front wheels of a vehicle. It has been knownfrom DE 197 28 419 A1 to determine the speed from an alternating signalcontained in the tangential acceleration signal supplied by theacceleration sensor and produced by the influence of gravitationalacceleration.

b is used here to describe the acceleration in the circumferentialdirection of a wheel encountered during acceleration or deceleration ofthe vehicle. The sign of the tangential acceleration signal b permits todistinguish between right and left wheels, and can be determined alreadyin the wheel-mounted electronic module and then transmitted to thereceiver. Although differentiation between right and left wheels is,preferable, effected in the acceleration phase following the start ofthe vehicle, it can generally be effected also in any acceleration ordeceleration phase.

The tyre-pressure monitoring systems obtainable in practice have thewheel-mounted electronic modules mounted on the wheel inside the tyreand have a hermetically sealed housing that does not permit the batteryto be exchanged. The battery is therefore expected to have a servicelife of 5 to 10 years. This requires the current consumption of thewheel-mounted electronic module to be minimised. For this purpose, ithas been known to provide the wheel-mounted electronic module with anintegrated circuit that activates the pressure sensor for measurementsonly during first time intervals and causes the transmitter to sendsignals during second time intervals longer than or equal to the firsttime intervals, i.e. in the first time intervals when a dangerouspressure loss has been detected, otherwise in the longer second timeintervals.

Known wheel-mounted electronic modules have a semiconductor-basedpressure sensor integrated into an integrated circuit. An accelerationsensor can also be integrated into such an integrated circuit. However,if according to DE 197 35 686 A1 two acceleration sensors or oneacceleration sensor sensitive in two axes perpendicular one to the otherare to be integrated into the integrated circuit in addition to thepressure sensor, then the development and production of the integratedcircuit becomes considerably more complex and expensive.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, the object of the present invention to open up a lessexpensive way of gaining signals in the wheel-mounted electronic modulefrom which conclusions can be drawn on the location of the wheel.

This object is achieved by a method having the features defined in claim1 and by a method having the features defined in claim 2. Advantageousfurther developments of the invention are the subject-matter of thesub-claims.

According to the invention, the speed of a wheel on which anacceleration sensor is mounted is determined as the frequency ω of analternating signal produced by the influence of the gravitationalacceleration g and contained in the acceleration signal b supplied bythe acceleration sensor. Alternatively, the object is achieved bydetermining the reciprocal value of the speed, instead of the speed, bydetermination of the period T of an alternating signal produced by theinfluence of the gravitational acceleration g, which is contained in theacceleration signal b supplied by the acceleration sensor.

The invention provides essential advantages:

For determining the speed of the wheel or its reciprocal value noseparate sensor is required; instead, these values are determined fromthe signals supplied by the provided acceleration sensor, whoseacceleration signal b has a component with a frequency ω, indicating thespeed of the wheel, produced by the influence of the gravitationalacceleration g. An acceleration sensor, which is designed and mounted onthe wheel so as to pick up any acceleration acting tangentially on thewheel, will be sufficient for this purpose.

The tangential acceleration signal b of such a simple accelerationsensor contains, by superposition, a speed-dependent alternating portionbecause the tangential acceleration b acting on the acceleration sensorwill be increased by the component of gravitational acceleration gacting in the tangential direction during downward movement of theacceleration sensor, while it will be decreased by the component ofgravitational acceleration g acting in the tangential direction duringupward movement of the acceleration sensor. The wheel speeds arepredominantly in a range of below 40 r.p.s. Accordingly, the frequencyof the alternating portion of the acceleration signal, produced bygravitational acceleration, is predominantly in a range below 40 Hz andcan be separated from the direct-voltage portion of the electricacceleration signal supplied by the acceleration sensor by filtering.

The determination of the angular frequency ω or the period T of anelectric alternating signal is easy and can be effected with highaccuracy since, basically, that process can be regarded as a timemeasurement and time measurements can be carried out more easily andwith higher accuracy than intensity measurements, which latter had to beperformed according to DE 197 35 686 A1 for determining the speed fromthe centrifugal acceleration.

The tangential acceleration b and the speed ω or the period T can bemeasured by means of a single acceleration sensor, which latter must besensitive only in the direction of a single axis, namely in thedirection in which the tangential acceleration occurs. Sensitivity in asecond axis or even a second acceleration sensor is not required.

For use in a tyre-pressure monitoring system it is, therefore, onlynecessary to integrate, in addition to the pressure sensor, a singleadditional acceleration sensor, sensitive to tangential acceleration,into the integrated circuit of the respective wheel-mounted electronicmodule.

The technical input for the development and production of the integratedcircuit can, therefore, be reduced.

The compound signal supplied by the acceleration sensor may be used,according to the method known from DE 197 35 686 A1 (see items 1. to 3.above) to allocate signals, that are sent out by the wheel-mountedelectronic modules when the vehicle is moving and that signal that thespeed is equal to zero, to a spare wheel and to differentiate betweensteered front wheels and unsteered rear wheels with the aid of anintegrated measurand of the speed or the period, because the integratedmeasurand of the speed will be greater for a steered front wheel thanfor an unsteered rear wheel and the integrated measurand of the periodwill be smaller for a steered front wheel than for an unsteered rearwheel. Finally, it is possible, as disclosed in DE 197 35 686 A1, todifferentiate between wheels on the right side of the vehicle and wheelson the left side of the vehicle by observing the sign of the tangentialacceleration (see item 3. above).

The wheel-mounted electronic modules of the different wheels areindependent one from the other so that normally they will not send outsignals at the same time. If the signals received from different wheelsare to be compared in the electric evaluation unit of the receiver inorder to determine the location in the vehicle of a wheel characterisedby a given identifier, then the signals, or signals derived therefrom inthe electronic evaluation unit, must be stored temporarily in theelectronic evaluation unit in order to permit such comparison. Theelectronic evaluation unit must be equipped for this purpose with avolatile storage suited for that purpose. The man skilled in the artwill know that this can be achieved by simple means, for example withthe aid of a microprocessor, which may additionally perform theevaluation of the signals received. When it is not necessary to comparethe signals arriving from different wheels, because the signals from agiven wheel-mounted electronic module can as such be clearly allocatedto a wheel location, then storing the signals temporarily will not beabsolutely necessary.

As the signals to be compared are not generated simultaneously, it mayhappen that a signal from a right-hand wheel, that has been producedwhile the vehicle was accelerated, is compared with a signal from aleft-hand wheel, that was generated while the vehicle was decelerated.In this case, it is not possible to distinguish between right and leftwheels from the sign of the tangential acceleration b alone. It istherefore proposed, according to a preferred further development of theinvention, to determine the variation over time of the speed ω or itsreciprocal value T simultaneously with the determination of thetangential acceleration b and its sign on each wheel, and to send suchvariation to the receiver together with the sign of the tangentialacceleration b. This is done because the sign of the variation over timedω/dt of the speed ω indicates if the vehicle was accelerated (positivesign) or decelerated (negative sign). Correspondingly, the sign of thevariation over time dT/dt of the period T indicates if the vehicle wasaccelerated (negative sign) or decelerated (positive sign). By jointlyevaluating the sign of the variation over time dω/dt of the speed ω anddT/dt of the period T and the sign of the tangential acceleration b foreach wheel one can now clearly differentiate between right wheels andleft wheels. To this end, one forms for each wheel in the centralelectronic evaluation unit of the receiver the product of those signs,which product will be opposite for wheels on the left side of thevehicle to the product of the signs for wheels on the right side of thevehicle, irrespective of whether the vehicle was accelerated ordecelerated. It is a requirement for this purpose that the accelerationsensors must be equally oriented relative to the respective wheel, whichcan be guaranteed for identical wheel-mounted electronic modules bymounting them in the same position on the several wheels. It is thenalso possible to determine in advance for the envisaged mountingposition which sign the product used for distinguishing between rightand left wheels is to have for wheels on the right side of the vehicleand for wheels on the left side of the vehicle.

Let it be assumed that the sign of the tangential acceleration b at aright wheel is positive when the vehicle is being accelerated; then thesign will be negative for a left wheel, and the sign of the variationover time dω/dt of the speed ω will be positive for the wheels on bothsides of the vehicle. The product of the signs then is

sign(dω/dt). sign b=(+1)(+1)=+1 for the right wheel and

 sign(dω/dt). sign b=(+1)(−1)=−1 for the left wheel.

From the above it results that the left wheel and the right wheeldistinguish themselves one from the other by the sign of the product.

Let it now be assumed that the signal on the right wheel was generatedwhile the vehicle was being accelerated, whereas the signal on the leftwheel was generated while the vehicle was being decelerated. The productof the signs then is equal to:

sign(dω/dt). sign b=(+1)(+1)=+1 for the right wheel and

sign(dω/dt). sign b=(−1)(+1)=−1 for the left wheel.

The left wheel and the right wheel can then be distinguished one fromthe other by the product of the signs in the same way as in the caseassumed first. The discrimination between the left and the right wheelis unambiguous, regardless of whether the vehicle was accelerated ordecelerated.

Once discrimination between the left and the right wheels has beeneffected in this way, when the vehicle has two axles, it is furtherpossible with the aid of two antennas, one of which is assigned to thewheels on the front axle of the vehicle, while the other one is assignedto the wheels on the rear axle of the vehicle, to determine which of theleft wheels is located in front and which is located at the rear andwhich of the right wheels is located in front and which is located atthe rear, by evaluating the intensity (reception amplitude) of thesignals received. This is so because an antenna arranged in theneighbourhood of the front axle will on average receive signalsoriginating from the front wheels with a higher amplitude than signalsoriginating from the rear wheels. Conversely, an antenna arranged nearthe rear axle will on average receive signals originating from the rearwheels with higher amplitude than signals originating from the frontwheels. For this purpose, the antennas need not even be arranged midwaybetween the right and the left wheels, but may be offset from thecentre, because discrimination between front wheels and rear wheels isin any case possible by combining the information on the sense ofrotation and the reception amplitude.

In the case of vehicles with more than two axles one can assign aseparate antenna to each axle.

In the case of a vehicle with two axles there is even the possibility todo with a single antenna provided the latter is arranged either closerto the front axle or closer to the rear axle, but in such a way thatsignals can be received by the antenna from all four wheels with asufficiently high amplitude. For determining the signal intensities,instead of using a single signal a sequence of a number of signalsoriginating from one and the same wheel can be used in order to improvethe accuracy of the intensity-determining process. As regards astatistical method suited for this purpose, reference is expressly madeto the disclosure of DE 196 08 478 A1 and DE 196 08 479 A1.

Under certain circumstances the alternating portion, resulting from theinfluence of the gravitational acceleration g, of the signal emitted bythe tangential acceleration sensor is not very clearly distinguishablefrom the signal background. Noise, vibrations and other interferinginfluences in the acceleration signal that may occur especially athigher driving speeds may result in one or the other determination ofthe speed of the wheel or of its reciprocal value and of its variationover time to be incorrect. In order to improve the reliability of thediscrimination between wheels on the right side and wheels on the leftside of the vehicle, it is proposed according to an advantageous furtherdevelopment of the invention to repeatedly form and integrate, insuccession over time, the product forming the criterion fordistinguishing between wheels on the right side and wheels on the leftside. Let it be assumed that the product formed as criterion has thevalue −1 in a particular case. If the product is always determinedcorrectly, the integration only has the effect to add the values −1 andthus results in a clearly negative value. If faulty measurements occur,these may lead to +1 values appearing in the sequence of −1 values.However, assuming plausibly that the correct values will be higher innumber, the integral will finally become clearly negative also in such acase affected by faults. The integration time used in any such case canbe fixed by experience so as to guarantee that the value of the integralwill in any case stabilise either on a positive or on a negative value.

In order to be able to distinguish between wheels on the right side ofthe vehicle and wheels on the left side of the vehicle, the only thingthat matters lastly is the sign formed as a criterion, which must bedetermined with sufficient reliability. The numerical value to which thesign is assigned is of no importance whatever. In determining theproduct of the signs of the tangential acceleration b and the variationover time dT/dt or the period T or the variation over time dω/dt of thespeed ω of the wheel, the signs may therefore be linked to with anynumerical values. To say it in other words: It is not necessary tonormalise the tangential acceleration b measured and the variation overtime dT/dt of the period T determined or the variation over time dω/dtof the speed ω determined to standard values prior to multiplying themfor the purpose of determining the sign of their product. Rather, theinstantaneous value of the tangential acceleration b obtained by themeasurement or evaluation step can be multiplied as it is with theinstantaneous value of the variation over time dT/dt of the period T orthe variation over time dω/dt of the speed ω of the wheel, giving dueconsideration to their signs, and can be integrated over time in orderto obtain a safe criterion for discriminating between right wheels andleft wheels.

BRIEF DESCRIPTION OF DRAWING FIGURES

FIG. 1 shows a flow diagram of the method according to the invention forthe discrimination between wheels on the right side of the vehicle andwheels on the left side of the vehicle; and

FIG. 2 shows, in diagrammatic form, the influence which thegravitational acceleration has on the measurement of the tangentialacceleration on one wheel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 the tangential acceleration b is initially determined for aselected wheel by the acceleration sensor in the wheel-mountedelectronic module. In a second step, the direct-current portion b= andthe alternating portion b˜ resulting from the influence of thegravitational acceleration g are separated by filtering the electricsignal for the tangential acceleration b emitted by the accelerationsensor. The period T of the alternating portion b˜ is determined in athird step. The variation over time dT/dt of the period T is determinedin a fourth step. In a fifth step, the direct-current portion b= ismultiplied by the variation over time dT/dt giving due consideration totheir signs. As a result of that multiplication, one obtains a value Dlinked to a sign which will be described hereafter as the instantaneoussense-of-rotation value. In a second step, the instantaneoussense-of-rotation value is integrated over a certain period of time sothat a safe sense-of-rotation value D is obtained:D = ∫_(t₁)^(t₂)D  t

The integration time t2−t1 may be firmly preselected, based onexperience, or may be flexibly selected so that the evaluation circuitperforming the integration will continuously check the integration valueto see if its sign stabilises, and will break off the integrationprocess when the sign has stabilised. From the stabilised sign it canthen be clearly concluded if the wheel observed is located on the rightside of the vehicle or on the left side of the vehicle.

The way in which the gravitational acceleration g takes influence on thetangential acceleration b measured is clearly obvious from therepresentation of FIG. 2. FIG. 2 shows a wheel 1 with a wheel-mountedelectronic module 42 mounted thereon in four different positionsdiffering each by a quarter of a revolution of the wheel. Gravitationalacceleration g always acts vertically in downward direction as indicatedby arrow 3. In its position 1 the wheel-mounted electronic module 2occupies its uppermost position. Gravitational acceleration g acts invertical direction relative to the tangential acceleration b in thisposition and does not change its value.

In the illustrated position 2, the wheel 1 has turned to the right by aquarter of a revolution. The tangential acceleration b encountered onthe wheel-mounted electronic module 2 is equally directed to thegravitational acceleration g in this case so that the tangentialacceleration b is instantaneously increased by the gravitationalacceleration g.

In the illustrated position 3 the wheel 1 has turned to the right by afurther quarter of a revolution, and the wheel-mounted electronic module2 occupies its lowermost position. The tangential acceleration bencountered on the wheel-mounted electronic module 2 actsperpendicularly to the gravitational acceleration g and is therefore notchanged by the latter.

In the illustrated position 4 the wheel 1 has turned to the right by anadditional quarter of a revolution. The tangential acceleration bencountered on the wheel-mounted electronic module 2 is oppositelydirected in this position to the gravitational acceleration g when thevehicle is being accelerated, so that the gravitational acceleration greduces the tangential acceleration b. The influence of thegravitational acceleration g on the tangential acceleration b isindicated in the small table in FIG. 1 by the measured values 0; 1 g; 0and −1 g.

The process step 1, namely determining the tangential acceleration b asa function of time, must be carried out in the respective wheel-mountedelectronic module 2. The further process steps are best carried out inthe central electronic evaluation unit of the receiver to which themeasured tangential acceleration b is transmitted in the form of radiosignals.

According to another advantageous further development of the invention,the speed ω of the wheel, that has been determined according to theinvention, is used for controlling the transmitter of the wheel-mountedelectronic module. This provides the possibility to activate thetransmitter at longer time intervals under less dangerous drivingconditions and at shorter time intervals under potentially moredangerous driving conditions. There is especially the possibility tocontrol the transmitter in such a way that normally the time intervals,at which signals are emitted, will be reduced as the speed of the wheelincreases. These time intervals, which are described in this contextalso as second time intervals, are normally longer than the first timeintervals at which measurements of the tyre pressure are effected by thewheel-mounted electronic module. As long as the measurements of the tyrepressure do not indicate that a dangerous situation exists (normalcase), it is not necessary that each non-dangerous measured value betransmitted to the receiver and be brought to the driver's knowledge. Insuch a normal case it will, therefore, be sufficient for thewheel-mounted electronic module to get into contact with the receiver atlonger second time intervals in order to indicate its serviceability.Conveniently, these second time intervals should be selected to belonger at lower speed and shorter at higher speed, which is renderedpossible in a simple way by determining the speed in the wheel-mountedelectronic module according to the invention. If the reciprocal value,namely the period T, is determined in the wheel-mounted electronicmodule instead of the speed ω, then the control of the second timeintervals of the transmitter can be effected in a corresponding way.

In dangerous cases, when the pressure sensor has determined that adangerous pressure deviation has occurred, the second time intervals canbe reduced, by deviation from the normal condition, to the first timeintervals and the first time intervals can be reduced relative to thenormal condition, in order to be able to detect and react to thepotentially dangerous pressure deviations, especially a rapid pressuredrop, by increased measuring and transmission rates.

What is claimed is:
 1. A method of determining the speed of a firstwheel and a second wheel on a motor vehicle by a signal produced by atangential acceleration sensor mounted on each wheel, and distinguishingthe signals produced by the acceleration sensor on said first wheel andthe acceleration sensor on said second wheel, the method comprising thesteps of: mounting said acceleration sensors in equal mounting positionson said wheels such that a sign of a tangential acceleration signalresulting from the motion state of said first wheel will be opposite asign of a tangential acceleration signal resulting from thecorresponding motion state of said second wheel; measuring thetangential acceleration signal for each wheel; determining the sign ofthe tangential acceleration signal for each wheel; measuring thefrequency of an alternating signal contained in the tangentialacceleration signal, for each wheel, the alternating signal beingproduced by the influence of gravitational force on said accelerationsensors; correlating the frequency of the alternating signals with thespeed of a wheel; determining a sign of a change in the frequency of thealternating signals over time for each wheel when the speed of thewheels varies; forming a product of the sign of the tangentialacceleration signal and the sign of the change in frequency of thealternating signal over time, for each wheel; and distinguishing thesignals of the acceleration sensors by the product of the signs.
 2. Themethod according to claim 1 further comprising the step of integratingover a period of time the product of the signs for each wheel, whichproduct has been formed given due consideration to the sign of thetangential acceleration signal and the sign of the change in frequencyof the alternating signal over time.
 3. The method according to claim 2wherein the integration step is continued until a sign of the value ofthe integral has stabilised.
 4. The method according to claim 1 furthercomprising the step of providing equipment for monitoring and signalingby radio any pressure change in pneumatic tires mounted on each of saidwheels, said equipment comprising: a current source, said accelerationsensor for determining the speed of the wheel, a pressure sensormeasuring the tyre pressure at first time intervals, and a transmitterwhich transmits at second time intervals, that are longer than or equalto the first time intervals, information on the measured tyre pressureas well as on the measured tangential acceleration signal to a receiverlocated in the vehicle, the second time intervals are controlled independence on the frequency of the respective wheel.
 5. A method ofdetermining the speed of a first wheel and a second wheel on a motorvehicle by a signal produced by a tangential acceleration sensor mountedon each wheel, and distinguishing the signals produced by theacceleration sensor on said first wheel and the acceleration sensor onsaid second wheel, the method comprising the steps of: mounting saidacceleration sensors in equal mounting positions on said wheels suchthat a sign of a tangential acceleration signal resulting from themotion state of said first wheel will be opposite a sign of a tangentialacceleration signal resulting from the corresponding motion state ofsaid second wheel; measuring the tangential acceleration signal for eachwheel; determining the sign of the tangential acceleration signal foreach wheel; measuring the period of an alternating signal contained inthe tangential acceleration signal, for each wheel, the alternatingsignal being produced by the influence of gravitational force on saidacceleration sensors; correlating the period of the alternating signalswith the speed of a wheel; determining a sign of a change in the periodof the alternating signals over time for each wheel when the speed ofthe wheels varies; forming a product of the sign of the tangentialacceleration signal and the sign of the change in period of thealternating signal over time, for each wheel; and distinguishing thesignals of the acceleration sensors by the product of the signs.
 6. Themethod according to claim 5 further comprising the step of integratingover a period of time the product of the signs for each wheel, whichproduct has been formed given due consideration to the sign of thetangential acceleration signal and the sign of the change in period ofthe alternating signal over time.
 7. The method according to claim 6wherein the integration step is continued until a sign of the value ofthe integral has stabilised.
 8. The method according to claim 5comprising the step of providing equipment for monitoring and signalingby radio any pressure change in pneumatic tires mounted on each of saidwheels, said equipment comprising: a current source, said accelerationsensor for determining the speed of the wheel, a pressure sensormeasuring the tyre pressure at first time intervals, and a transmitterwhich transmits at second time intervals, that are longer than or equalto the first time intervals, information on the measured tyre pressureas well as on the measured tangential acceleration signal to a receiverlocated in the vehicle, the second time intervals are controlled independence on the period of the respective wheel.